Chapter 7: Deadlocks. Operating System Concepts 9 th Edition
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1 Chapter 7: Deadlocks Silberschatz, Galvin and Gagne 2013
2 Chapter 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock 7.2 Silberschatz, Galvin and Gagne 2013
3 Chapter Objectives In mutliprogramming environment Several processes compete for finite number of resources Each process can request a resource and wait until it is available Deadlock situation Process P i requests resource R b held by process P j Process P i is then waiting for P j to release resource R b In the meantime, Process P j requests resource R a held by process P i Process P j is then waiting for P i to release resource R a P i can t release R a because it needs R b in order to release it P j can t release R b because it needs R a in order to release it To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of different methods for preventing or avoiding deadlocks in a computer system 7.3 Silberschatz, Galvin and Gagne 2013
4 System Model System consists of finite numbers of resources Resource types R 1, R 2,..., R m CPU cycles, memory space, I/O devices, files, data, locks, etc Each resource type R i has W i instances. If a system has two CPUs then resource type CPU has two instances Each process utilizes a resource as follows: request ; a process must request first then wait until it can acquire resource use ; the process operates on the resource release ; the process releases the resource Number of resources requested cannot exceed number of resources in system A set of processes is in a deadlocked state if each process in the set is waiting for an event that can be cause only by another process in the set Processes never finish executing; preventing other processes from starting 7.4 Silberschatz, Galvin and Gagne 2013
5 Deadlock Characterization Necessary conditions: deadlock can arise if four conditions hold simultaneously. Mutual exclusion: only one process at a time can use a resource; that is, at least one resource must be held in non-sharable mode. Requesting process must wait until resource is released Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task; that is, resources cannot be preempted Circular wait: there exists a set {P 0, P 1,, P n } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2,, P n 1 is waiting for a resource that is held by P n, and P n is waiting for a resource that is held by P 0. All four conditions must hold for a deadlock to occur 7.5 Silberschatz, Galvin and Gagne 2013
6 Resource-Allocation Graph Deadlock Description: A set of vertices V and a set of edges E. V is partitioned into two types: P = {P 1, P 2,, P n }, the set consisting of all the processes in the system R = {R 1, R 2,, R m }, the set consisting of all resource types in the system request edge directed edge P i R j Process P i has requested an instance of resource type R j And P i is currently waiting for that resource assignment edge directed edge R j P i Instance of R j has been allocated to process P i 7.6 Silberschatz, Galvin and Gagne 2013
7 Resource-Allocation Graph Process; circles Resource Type with 4 instances; rectangles with a dot for each instance P i requests instance of R j P i P i is holding an instance of R j R j P i R j 7.7 Silberschatz, Galvin and Gagne 2013
8 Example of a Resource Allocation Graph 7.8 Silberschatz, Galvin and Gagne 2013
9 Basic Facts If graph contains no cycles no process in the system is deadlocked If graph contains a cycle a deadlock may exist if only one instance per resource type, then a deadlock has occurred Cycle is necessary and sufficient condition for deadlock if several instances per resource type, possibility of deadlock Cycle is necessary but not sufficient condition for deadlock 7.9 Silberschatz, Galvin and Gagne 2013
10 Resource Allocation Graph With A Deadlock 7.10 Silberschatz, Galvin and Gagne 2013
11 Graph With A Cycle But No Deadlock No deadlock: since P 4 may release its instance of resource type R 2 which can be allocated to P 3 ; thus breaking the cycle 7.11 Silberschatz, Galvin and Gagne 2013
12 Methods for Handling Deadlocks Devise a protocol ensuring that the system will never enter a deadlocked state: Deadlock prevention scheme Ensure that at least one of the four necessary conditions cannot hold Prevent deadlocks by constraining how requests can be made Deadlock avoidance scheme OS use additional knowledge: Which resources a process will request in its lifetime Currently available resources, and, currently allocated resources Future release of each process Then can decide If a process should wait, or, if a request can be satisfied Allow the system to enter a deadlock state and then recover Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including Linux, Windows, and UNIX 7.12 Silberschatz, Galvin and Gagne 2013
13 Deadlock Prevention Restrain the ways request can be made Mutual Exclusion not required for sharable resources (e.g., read-only files); must hold for non-sharable resources Sharable resources cannot be involved in a deadlock; wait never needed But: mutex locks cannot be simultaneously shared by several process Hold and Wait must guarantee that whenever a process requests a resource, it does not hold any other resources Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none allocated to it. Low resource utilization; starvation possible 7.13 Silberschatz, Galvin and Gagne 2013
14 Deadlock Prevention No Preemption If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released Preempted resources are added to the list of resources for which the process is waiting Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting Circular Wait impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration Request resources according to the decided order or request Release lower-order resources before requesting any high-order resource 7.14 Silberschatz, Galvin and Gagne 2013
15 Deadlock Example /* thread one runs in this function */ void *do_work_one(void *param) { } pthread_mutex_lock(&first_mutex); pthread_mutex_lock(&second_mutex); /** * Do some work */ pthread_mutex_unlock(&second_mutex); pthread_mutex_unlock(&first_mutex); pthread_exit(0); /* thread two runs in this function */ void *do_work_two(void *param) { } pthread_mutex_lock(&second_mutex); locks should not be requested in this order pthread_mutex_lock(&first_mutex); /** * Do some work */ pthread_mutex_unlock(&first_mutex); pthread_mutex_unlock(&second_mutex); pthread_exit(0); 7.15 Silberschatz, Galvin and Gagne 2013
16 Deadlock Example with Lock Ordering void transaction(account from, Account to, double amount) { mutex lock1, lock2; lock1 = get_lock(from); lock2 = get_lock(to); acquire(lock1); acquire(lock2); withdraw(from, amount); deposit(to, amount); release(lock2); release(lock1); } Transactions 1 and 2 execute concurrently. Transaction 1 transfers $25 from account A to account B, and Transaction 2 transfers $50 from account B to account A 7.16 Silberschatz, Galvin and Gagne 2013
17 End of Chapter 7 Silberschatz, Galvin and Gagne 2013
18 Deadlock with Mutex Locks Deadlocks can occur via system calls, locking, etc. See example box in text page 318 for mutex deadlock 7.18 Silberschatz, Galvin and Gagne 2013
19 Deadlock Avoidance Requires that the system has some additional a priori information available Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes 7.19 Silberschatz, Galvin and Gagne 2013
20 Safe State When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state System is in safe state if there exists a sequence <P 1, P 2,, P n > of ALL the processes in the systems such that for each P i, the resources that P i can still request can be satisfied by currently available resources + resources held by all the P j, with j < I That is: If P i resource needs are not immediately available, then P i can wait until all P j have finished When P j is finished, P i can obtain needed resources, execute, return allocated resources, and terminate When P i terminates, P i +1 can obtain its needed resources, and so on 7.20 Silberschatz, Galvin and Gagne 2013
21 Basic Facts If a system is in safe state no deadlocks If a system is in unsafe state possibility of deadlock Avoidance ensure that a system will never enter an unsafe state Silberschatz, Galvin and Gagne 2013
22 Safe, Unsafe, Deadlock State 7.22 Silberschatz, Galvin and Gagne 2013
23 Avoidance Algorithms Single instance of a resource type Use a resource-allocation graph Multiple instances of a resource type Use the banker s algorithm 7.23 Silberschatz, Galvin and Gagne 2013
24 Resource-Allocation Graph Scheme Claim edge P i R j indicated that process P j may request resource R j ; represented by a dashed line Claim edge converts to request edge when a process requests a resource Request edge converted to an assignment edge when the resource is allocated to the process When a resource is released by a process, assignment edge reconverts to a claim edge Resources must be claimed a priori in the system 7.24 Silberschatz, Galvin and Gagne 2013
25 Resource-Allocation Graph 7.25 Silberschatz, Galvin and Gagne 2013
26 Unsafe State In Resource-Allocation Graph 7.26 Silberschatz, Galvin and Gagne 2013
27 Resource-Allocation Graph Algorithm Suppose that process P i requests a resource R j The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph 7.27 Silberschatz, Galvin and Gagne 2013
28 Banker s Algorithm Multiple instances Each process must a priori claim maximum use When a process requests a resource it may have to wait When a process gets all its resources it must return them in a finite amount of time 7.28 Silberschatz, Galvin and Gagne 2013
29 Data Structures for the Banker s Algorithm Let n = number of processes, and m = number of resources types. Available: Vector of length m. If available [j] = k, there are k instances of resource type R j available Max: n x m matrix. If Max [i,j] = k, then process P i may request at most k instances of resource type R j Allocation: n x m matrix. If Allocation[i,j] = k then P i is currently allocated k instances of R j Need: n x m matrix. If Need[i,j] = k, then P i may need k more instances of R j to complete its task Need [i,j] = Max[i,j] Allocation [i,j] 7.29 Silberschatz, Galvin and Gagne 2013
30 Safety Algorithm 1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i = 0, 1,, n Find an i such that both: (a) Finish [i] = false (b) Need i Work If no such i exists, go to step 4 3. Work = Work + Allocation i Finish[i] = true go to step 2 4. If Finish [i] == true for all i, then the system is in a safe state 7.30 Silberschatz, Galvin and Gagne 2013
31 Resource-Request Algorithm for Process P i Request i = request vector for process P i. If Request i [j] = k then process P i wants k instances of resource type R j 1. If Request i Need i go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim 2. If Request i Available, go to step 3. Otherwise P i must wait, since resources are not available 3. Pretend to allocate requested resources to P i by modifying the state as follows: Available = Available Request i ; Allocation i = Allocation i + Request i ; Need i = Need i Request i ; If safe the resources are allocated to P i If unsafe P i must wait, and the old resource-allocation state is restored 7.31 Silberschatz, Galvin and Gagne 2013
32 Example of Banker s Algorithm 5 processes P 0 through P 4 ; 3 resource types: A (10 instances), B (5instances), and C (7 instances) Snapshot at time T 0 : Allocation Max Available A B C A B C A B C P P P P P Silberschatz, Galvin and Gagne 2013
33 Example (Cont.) The content of the matrix Need is defined to be Max Allocation Need A B C P P P P P The system is in a safe state since the sequence < P 1, P 3, P 4, P 2, P 0 > satisfies safety criteria 7.33 Silberschatz, Galvin and Gagne 2013
34 Example: P 1 Request (1,0,2) Check that Request Available (that is, (1,0,2) (3,3,2) true Allocation Need Available A B C A B C A B C P P P P P Executing safety algorithm shows that sequence < P 1, P 3, P 4, P 0, P 2 > satisfies safety requirement Can request for (3,3,0) by P 4 be granted? Can request for (0,2,0) by P 0 be granted? 7.34 Silberschatz, Galvin and Gagne 2013
35 Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme 7.35 Silberschatz, Galvin and Gagne 2013
36 Single Instance of Each Resource Type Maintain wait-for graph Nodes are processes P i P j if P i is waiting for P j Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock An algorithm to detect a cycle in a graph requires an order of n 2 operations, where n is the number of vertices in the graph 7.36 Silberschatz, Galvin and Gagne 2013
37 Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph 7.37 Silberschatz, Galvin and Gagne 2013
38 Several Instances of a Resource Type Available: A vector of length m indicates the number of available resources of each type Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process Request: An n x m matrix indicates the current request of each process. If Request [i][j] = k, then process P i is requesting k more instances of resource type R j Silberschatz, Galvin and Gagne 2013
39 Detection Algorithm 1. Let Work and Finish be vectors of length m and n, respectively Initialize: (a) Work = Available (b) For i = 1,2,, n, if Allocation i 0, then Finish[i] = false; otherwise, Finish[i] = true 2. Find an index i such that both: (a) Finish[i] == false (b) Request i Work If no such i exists, go to step Silberschatz, Galvin and Gagne 2013
40 Detection Algorithm (Cont.) 3. Work = Work + Allocation i Finish[i] = true go to step 2 4. If Finish[i] == false, for some i, 1 i n, then the system is in deadlock state. Moreover, if Finish[i] == false, then P i is deadlocked Algorithm requires an order of O(m x n 2 ) operations to detect whether the system is in deadlocked state 7.40 Silberschatz, Galvin and Gagne 2013
41 Example of Detection Algorithm Five processes P 0 through P 4 ; three resource types A (7 instances), B (2 instances), and C (6 instances) Snapshot at time T 0 : Allocation Request Available A B C A B C A B C P P P P P Sequence <P 0, P 2, P 3, P 1, P 4 > will result in Finish[i] = true for all i 7.41 Silberschatz, Galvin and Gagne 2013
42 Example (Cont.) P 2 requests an additional instance of type C State of system? Request A B C P P P P P Can reclaim resources held by process P 0, but insufficient resources to fulfill other processes; requests Deadlock exists, consisting of processes P 1, P 2, P 3, and P Silberschatz, Galvin and Gagne 2013
43 Detection-Algorithm Usage When, and how often, to invoke depends on: How often a deadlock is likely to occur? How many processes will need to be rolled back? one for each disjoint cycle If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes caused the deadlock Silberschatz, Galvin and Gagne 2013
44 Recovery from Deadlock: Process Termination Abort all deadlocked processes Abort one process at a time until the deadlock cycle is eliminated In which order should we choose to abort? 1. Priority of the process 2. How long process has computed, and how much longer to completion 3. Resources the process has used 4. Resources process needs to complete 5. How many processes will need to be terminated 6. Is process interactive or batch? 7.44 Silberschatz, Galvin and Gagne 2013
45 Recovery from Deadlock: Resource Preemption Selecting a victim minimize cost Rollback return to some safe state, restart process for that state Starvation same process may always be picked as victim, include number of rollback in cost factor 7.45 Silberschatz, Galvin and Gagne 2013
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